Ultrasonic transducers provide a means for converting electrical energy into acoustic energy and vice versa. When the electrical energy is in the form of an RF signal, a correctly designed transducer can produce ultrasonic signals with the same frequency characteristics as the driving electrical RF signal. Diagnostic ultrasound has traditionally been used at center frequencies ranging from less than 1 MHz to about 10 MHz. One skilled in the art will understand that this frequency spectrum provides a means of imaging biological tissue with resolution ranging from several mm to generally greater than 300 um and at depths from a few mm down to 10 s of cm.
High frequency ultrasonic transducers are generally ultrasonic transducers with center frequencies above 15 MHz and ranging to over 60 MHz. High frequency ultrasonic transducers provide higher resolution while limiting the maximum depth of penetration, and as such, provide a means of imaging biological tissue from a depth of a fraction of a mm to over 3 cm with resolutions in the 20 um to over 300 um range.
There are many challenges associated with fabricating high frequency ultrasonic transducers that do not arise when working with traditional clinical ultrasonic transducers that operate at frequencies below about 10 MHz. One skilled in the art will understand that structures generally scale down according to the inverse of the frequency, so that a 50 MHz transducer will have structures about 10 times smaller than a 5 MHz transducer. In some cases, materials or techniques cannot be scaled down to the required size or shape, or in doing so they lose their function and new technologies must be developed or adapted to allow high frequency ultrasonic transducers to be realized. In other cases, entirely new requirements exist when dealing with the higher radio frequency electronic and acoustic signals associated with HFUS transducers.
RF electrical interconnections require that some form of transmission line be employed to effectively contain the magnetic fields surrounding the signal and ground conductors. One skilled in the art will appreciate that depending on the frequencies being transmitted, and the length of the conductors, electrical impedance matching and shielding techniques must be employed for optimal performance. One skilled in the art will further appreciate that at lower clinical frequencies, such interconnections are highly developed and available in a wide variety of options to the ultrasound system and transducer designers and that such interconnections typically consist of several components as follows: First, a connection to the ultrasound system, which typically consists of a zero insertion force (ZIF) type or other large format connector; second, the electrical cables running from the system connector toward the transducer (typically micro-coaxial transmission lines); third, an interface between the cables and the transducer usually including connectors and/or a printed circuit board; and finally, an interface from the connector or circuit board to each of the transducer elements. This typical set of components is readily available in the industry, with many variations being successfully employed for traditional clinical frequency US transducers.
One skilled in the art will appreciate that some of these components will readily scale to the higher frequencies associated with HFUS and other will not. Micro-coaxial transmission lines are well suited to the higher frequencies associated with HFUS, and many industry standard connector solutions are applicable at the system end as well. Furthermore, one skilled in the art will know that printed circuit boards can be designed to function at orders of magnitude higher frequencies than those required for HFUS. The challenge for electrically interconnecting HFUS transducers to the ultrasound system lies principally in the means of making electrical connections to the actual elements of the HFUS array. These elements are very small, fragile, and often limited to strict thermal budgets so that traditional micro electrical interconnection techniques are not suitable for HFUS transducers. Wire bonding, low temperature soldering, and ACF adhesives for example, are widely used technologies for making interconnections to traditional clinical frequency transducers. However, there are limitations to these techniques that make them generally unsuitable for use on HFUS transducers. For example, one skilled in the art will appreciate that wire bonding of interconnections at pitches less than about 100 um can be challenging, and at pitches below 50 um become nearly impossible. When process temperature are limited to less than about 100 degrees C., wire bonding is even more challenging. In addition, mechanical forces associated with wire bonding become problematic when substrate thickness is less than about 100 um. Typical piezoelectric materials suitable for making HFUS transducers must be thinned to about 100 um down to less than 30 um for transducers spanning the 15 MHz to 50 MHz center frequency range. These thin substrates tend to crack when wire bonding is attempted. ACF tape and other asymmetrical conductive adhesive systems are not suitable for high reliability connections at pitches below about 200 um, and also generally require a thermal budget in excess of 120 degrees Celsius, which one skilled in the art will understand, may problematic for some materials associated with the fabrication of HFUS transducer materials.
Some HFUS transducers currently employ a grounding system that relies on a copper electrode made from thin conductive foil to be electrically attached to the front (lens side) ground plane of the transducer, and then exit the side of the stack and wrap around toward the flex circuits ground planes.
The primary challenge of this approach is related to the spacing of the lens to the ground plane of the piezoelectric material. In the conductive foil design, this space is equal to the thickness of the matching layers between the piezoelectric substrate and the lens, for example, in a three matching layer device, three quarter wave matching layers or about 30 um at 50 MHz up to about 70 um for 20 MHz (for reference, typical printer paper is 100 um thick). This necessitates the use of very thin foil attached to the array ground plane with a very thin bond line of conductive epoxy. Preservation of the mechanical integrity of the foil during subsequent lapping and adhesive/cleaning procedures is very challenging. Other methods might be employed to allow the use of thicker foil, but a secondary limitation of the foil is the risk of causing a delamination of the lens due to forces associated with bending the conductive foil, which are increased as the foil becomes thicker.
Finally, one skilled in the art will recognize that the conductive foil technique requires that the ground electrode exits the stack structure along the edges, making electrical isolation of the device challenging especially when BF or CF medical device ratings are required. Given these problems, there is a need for improved techniques of making connections to high frequency ultrasound transducer elements.